1. Technical Field
The present invention relates to a partially carbonitriding heat treated stainless steel ferrule, and more particularly, to a partially carbonitriding heat treated stainless steel ferrule, wherein the surface of a stainless steel ferrule is subjected to carbonitriding treatment, so that a multilayer structure is partially produced and corrosion resistance is maintained.
2. Description of the Related Art
Heat treatment is widely utilized to enhance surface hardness of a workpiece. A surface hardening method with regard to heat treatment includes a physical surface hardening method and a chemical surface hardening method. Nitriding or carburizing is a representative chemical surface hardening method, which causes the chemical component of a base metal to change to achieve surface hardening. In a typical chemical surface hardening method, heat treatment is performed by bringing a workpiece into contact with a gas or molten salt solution for carburizing or nitriding at high temperature to thus diffuse the carbon or nitrogen atoms to the surface of the workpiece. Thereby, the carburizing or nitriding process enables a compound layer having high hardness to be formed on the surface of the workpiece. It is known that in the carburizing or nitriding process, the transformation of the base metal is very low and wear resistance, corrosion resistance and thermal stability of the hardening layer are superior, compared to other surface hardening methods.
Salt bath heat treatment may be used to increase hardness of a metal having high corrosion resistance such as iron (stainless steel) containing chromium. A compound layer for increasing hardness is typically provided via formation of a nitride precipitate or a carbide precipitate abbreviated to “nitride” or “carbide”.
As such, a carbide (Cr3C2) is configured such that chromium is precipitated with respect to carbon. When a non-uniform surface structure is formed through precipitation in this way, a difference in electronegativity may occur between a portion where chromium is lacking due to removal of Cr and a portion where Cr is precipitated. Such a difference allows for action as a kind of galvanic cell, and thus a metal product may easily corrode. The precipitation easily takes place when the same element as in the precipitate is present in a large amount in the workpiece or it is easy to permeate particles due to high heat treatment temperature.
The case where the carbide is formed in a smaller amount on the surface of the workpiece is favorable but is difficult to control.
Also, a cyanide compound (HCN, KCN, etc.) for general use in carbonitriding is very harmful to the human body, and alternative materials thereto have to be found.
Although a workpiece needs to be totally hardened, a part such as a ferrule is required to have high hardness only on a portion thereof.
FIGS. 1A and 1B are a cross-sectional view and a perspective view, respectively, illustrating a ferrule having a predetermined shape used to connect two pipes.
FIG. 1A is a cross-sectional view illustrating the connection structure of two pipes using a ferrule.
When the two pipes, for example, a front pipe 11 and a rear pipe 15 are connected, the ferrule functions to close a gap between the front pipe 11 and the rear pipe 15 and is responsible for swaging the pipes to ensure a sealing function and for preventing separation of the pipes.
The ferrule may include a front ferrule 13 and a back ferrule 14.
The back ferrule 14 plays a role in that while the rear (tail portion 14a) of the back ferrule 14 is pushed by a nut 12 for tightening the pipes, a force is transferred to the front ferrule 13. As such, while the nut 12 rotates, it transfers the force to the ferrules to tighten the pipes, and thus rotational torque is created. The back ferrule 14 performs a linear motion in the travel direction of the nut 12 when the nut 12 is moved forward while rotating.
By the back ferrule 14, the lower beveled portion of the rear of the front ferrule 13 is lifted up, and a nose portion 14b swages the pipes to thereby prevent the separation of the pipes.
Accordingly, desired purposes may be achieved only when the hardness of the nose portion 14b of the back ferrule 14 is high. If the entire back ferrule 14 has high hardness, rotational torque of the nut 12 is not efficiently absorbed, thus increasing brittleness. Thus, only the nose portion 14b of the back ferrule 14 has to be selectively hardened.
FIG. 1B is a perspective view illustrating the ferrule. The ferrule is ring-shaped and the nose portion 14b thereof is transformed by a force applied to the front from the back. Hence, the nose portion 14b needs to be particularly hardened.
The back ferrule 13 includes a support portion 14a to which pressure is applied while the nut 12 is tightened, and a nose portion 14b which receives the applied pressure and thus undergoes irreversible transformation and closes and swages the edge of the pipe. As high friction and force are applied in the course of transformation, the nose portion 14b has to possess high hardness and elasticity. Accordingly, a part that selectively requires high hardness on a predetermined portion, such as the ferrule 14, should undergo selective partial hardening treatment.
If the entire back ferrule 14 is subjected to hardening treatment to create high hardness, irreversible transformation for swaging the pipes needs greater force, which results in that rotational torque on the nut 12 may further increase, undesirably incurring poor workability.
When high hardness is required only on a predetermined portion in this way, such a portion is hardened through partial heat treatment. Typically, a partial heat treatment method includes plating a workpiece with a different kind of metal, wherein the resulting plating is used as a mask against heat treatment. Specifically, the workpiece is plated with a different kind of metal, and the plating is removed from a portion to be hardened, so that the surface of the workpiece is externally exposed. Then, heat treatment is performed, and thereby the portion which is not externally exposed blocks the permeation of nitrogen or carbon due to the plating, and thus precipitation does not easily occur. Consequently, only the exposed portion is selectively hardened.
However, upon long-term heat treatment, chromium is precipitated on the portion which undergoes the heat treatment, remarkably deteriorating corrosion resistance, which is undesirable.